When do you use Marcus's and Marcus'?

Answer 1

Both are correct singular possessive forms.

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the word: Marcus'

Add an apostrophe s ('s) after the existing s at the end of the word: Marcus's

Examples:

Marcus' bicycle is new.

Marcus's bicycle is new.

Answer 2

Both "Marcus's" and "Marcus'" are correct forms to show possession for the name Marcus. "Marcus's" is used when indicating singular possession, while "Marcus'" is used when indicating plural possession or to avoid awkward pronunciation when adding another sibilant sound to the possessive form.